TABLE OF CONTENTS When a transistor is used as an amplifier, it is said to be operating in the active mode. Since a transistor is a 3-terminal device, there are three currents, the base current, denoted as i B, [1] the collector current, denoted as i C, and the emitter current, denoted as i E. They are shown in Figures 3.5 and 3.6.
For any transistor, NPN or PNP, the three currents are related as
| (3.1) |  |
We recall from Chapter 2, equation (2.3), that 
In a transistor, n ? 1, and the collector current is
| (3.2) |  |
where I r is the reverse (saturation) current, typically 10 -12 A to 10 -15 A as in junction diodes, v BE is the base-to-emitter voltage, and V T ? 26 mV at T = 300 K.
A very useful parameter in transistors is the common-emitter gain ?, a constant whose value typically ranges from 75 to 300. Its value is specified by the manufacturer. Please refer to Section 3.19. The base current i B is much smaller than the collector current i C and these two currents are related in terms of the constant ? as
| (3.3) |  |
and with (3.2) we get
| (3.4) |  |
From (3.1) and (3.3) we get
| (3.5) |  |
and with (3.2)
| (3.6) |  |
Another important parameter in transistors is the common-base
